1. Field of the Invention
The present invention relates to a solid-state image sensing device having photoelectric converting means for outputting a signal proportional to a logarithm value of an amount of incident light.
2. Description of the Related Art
A solid-state image sensing device is small, light weight and of low-power consumption. Moreover, image distortion and burning do not occur in the device, and the device is strong in environmental conditions such as vibration and magnetic field. Further, since the device can be manufactured by the step which is common with or similar to that of LSI (Large Scale Integrated Circuit), its reliability is high and it is suitable to mass production. For this reason, a solid-state image sensing device where pixels are arranged linearly is used widely for a facsimile and a flat bed scanner, and a solid-state image sensing device where pixels are arranged in a matrix pattern is used widely for a video camera and a digital camera. Such solid-state image sensing devices are roughly classified into CCD type and MOS type devices according to means for reading (taking out) photoelectric charges generated from a photoelectric converting element. A CCD type device stores photoelectric charges in a potential well and simultaneously transmits them, thus arising a disadvantage that a dynamic range is narrow. On the other hand, an MOS type device reads electric charges stored in a pn junction capacitance of a photodiode via an MOS transistor.
As a technique for widening a dynamic range of an MOS type solid-state image sensing device, U.S. Pat. No. 5,241,575 which is assigned to the alienee of this patent application is known. The U.S. Pat. No. 5,241,575 suggests a solid-state image sensing device which includes photocurrent generating means for generating a photocurrent according to an amount of incident light, an MOS transistor for inputting a photocurrent, and bias means for biasing the MOS transistor so that a sub-threshold current flows in the MOS transistor. In this device, a photocurrent is logarithmically converted.
Although such a solid-state image sensing device has a wide dynamic range, threshold properties of the MOS transistors provided for respective pixels are different from one another, thereby occasionally making sensitivity different per pixel. Therefore, it is necessary to hold an output obtained by previously emitting a bright light (uniform light) with uniform brightness as correcting data for correcting outputs of respective pixels at the time of taking an image of an object.
However, there arises problems such that it is complicated for an operator to irradiate the respective pixels using an external light source and exposure cannot be executed uniformly and effectively. Moreover, when an uniform light emitting mechanism is provided to an image sensing device, there arises a problem that the structure of the image sensing device becomes complicated. Therefore, as a technique which solves this problem, Japanese Patent Application Laid-Open No. 2001-094878 which is assigned to the alienee of this patent application is known. This Japanese patent publication suggests a solid-state image sensing device which is capable of counteracting sensitivity unevenness of the respective pixels without previously emitting an uniform light.
FIG. 1 shows a structure of pixel provided to the solid-state image sensing device disclosed in this Japanese patent publication. In the pixels in FIG. 1, a pn photodiode PD forms a photosensitive section (photoelectric converting section). An anode of the photodiode PD is connected with a drain of an MOS transistor T1, and a source of the MOS transistor T1 is connected with a drain and a gate of an MOS transistor T2 and a gate of an MOS transistor T3. A source of the MOS transistor T3 is connected with a drain of an MOS transistor T4 for selecting lines. A source of the MOS transistor T4 is connected with an output signal line 6. Here, the MOS transistors T1 through T4 are N-channel MOS transistors, respectively, and are grounded with back gate.
In addition, a DC voltage VPD is applied to a cathode of the photodiode PD and a drain of the MOS transistor T3. On the other hand, a signal φVPS is input into a source of the MOS transistor T2. Moreover, a signal φS is input into a gate of the MOS transistor T1, and a signal φV is input into a gate of the MOS transistor T4.
Respective signals are given to the pixels having such a structure according to a timing chart shown in FIG. 15. Namely, the signal φS is brought into high level so that the MOS transistor T1 is turned ON, and the signal φVPS is brought into high level and a bias voltage to be applied to the MOS transistor T2 is set so that the MOS transistor T2 is operated in a sub-threshold region. Thereafter, a pulse signal φV is given thereto so that a signal at the time of image sensing is output as image data.
The signal φS is brought into low level so that the MOS transistor T1 is turned OFF. The signal φVPS is brought into low level so that a bias voltage to be given to the MOS transistor T2 is different from that at the time of image sensing, thereby resetting the MOS transistor T2. Thereafter, the signal φVPS is brought into high level. The pulse signal φV is given in this state, so that a signal which reflects a threshold voltage of the MOS transistor T2 is output. This signal represents a sensitivity unevenness between the pixels and is used as correcting data for correcting sensitivity unevenness. Thereafter, the signal φS is brought into high level so that an image sensing operation is performed. Image data obtained in such a manner are corrected by the correcting data so that the sensitivity unevenness between the pixels can be canceled.
However, in the solid-state image sensing device having the function for canceling sensitivity unevenness, it is found that there is a fear of after-image phenomenon. According to examination by the inventors, it is supposed that this phenomenon occurs due to the following reasons.
When the solid-state image sensing device having the function for canceling sensitivity unevenness performs the reset operation as mentioned above, the MOS transistor T1 is turned OFF so that electric connection between the anode of the photodiode PD and the drain of the MOS transistor T2 is cut. Therefore, the MOS transistor T2 is reset, but electric charges stored in the drain of the MOS transistor T1 and the anode of the photodiode PD remain. Since the image sensing operation is performed in the state that electric charges remain, it is considered that the after-image phenomenon occurs. The after-image phenomenon occurs remarkably at the time of low brightness.